Earthquakes

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Earthquakes
Chiwaukee Academy Enhance K-12 Earth Science
Teaching through Hands-on Exercises
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What is an Earthquake?

• An earthquake is the vibration of Earth produced
  by the rapid release of energy
• Energy released radiates in all directions from
  its source, the focus
• Epicenter surface location directly above the
  focus
• Energy is in the form of waves
• Sensitive instruments around the world record the
  event

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Earthquake Focus Epicenter
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Earthquake Destruction

• Amount of structural damage attributable to
  earthquake vibrations depends on
• Intensity and duration of the vibrations
• Nature of the material upon which the structure
  rests
• Design of the structure

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Notable Earthquakes
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1906 San Francisco, CA 7.8 M, 1500 Deaths
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1964 Anchorage, AK9.2 M, 131 Deaths
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1989 Loma Prieta
Mission District San Francisco California 6.9 M,
62 Deaths
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What is an Earthquake?

• Earthquakes and faults
• Earthquakes usually associated with large
  fractures in Earths crust called faults
• Most of the motion along faults can be explained
  by the plate tectonics theory
• Earthquake mechanism
• Slippage at the weakest point (the focus) occurs
• Vibrations (earthquakes) occur as the deformed
  rock springs back to its original shape
  (elastic rebound)

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What is an Earthquake?

• Elastic rebound
• Rocks on both sides of an existing fault are
  deformed by tectonic forces
• Rocks bend and store elastic energy
• Frictional resistance holding the rocks together
  is overcome rock fractures (breaks) resulting
  in an earthquake
• Earthquakes most often occur along existing
  faults whenever the frictional forces on the
  fault surfaces are overcome

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Elastic Rebound
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What is an Earthquake

• Foreshocks and Aftershocks
• Aftershocks smaller earthquakes due to
  adjustments that follow a major earthquake
• Foreshocks small earthquakes that often precede
  a major earthquake
• days or, in some cases
• several years

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San Andreas Fault SystemAn active earthquake zone

• Most studied fault system in the world
• Displacement occurs along discrete segments 100
  to 200 km long
• Some portions exhibit slow, gradual displacement
  known as fault creep
• Other segments regularly slip producing small
  earthquakes
• Some segments store elastic energy for hundreds
  of years before rupturing in great earthquakes,
  strike slip motion

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San Andreas Fault
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San Andreas Fault(view north, Tomales Bay, CA
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Displacement from 1906 San Francisco Earthquake
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Seismology

• Seismology study of earthquake waves
• dates back almost 2000 years to the Chinese
• allows study of Earths interior
• Seismographs instrument that record seismic
  waves
• Records the movement of Earth in relation to a
  stationary mass on a rotating drum or magnetic
  tape
• horizontal motion
• vertical motion
• Seismograph record is a seismogram

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Seismograph (measures horizontal ground motion)
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Seismograph (measures vertical ground motion)
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Seismology

• Types of seismic waves
• Surface waves
• Travel along outer part of Earth
• Complex motion
• Cause greatest destruction
• Waves exhibit greatest amplitude and slowest
  velocity
• Waves have the greatest periods (time interval
  between crests)
• Often referred to as long waves, or L waves
• Body waves

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Seismology

• Body waves
• Travel through Earths interior
• Two types based on mode of travel
• Primary (P) waves
• Push-pull (compress and expand) motion, change
  the volume of intervening material
• Travel through solids, liquids, and gases
• Generally, in any solid material, P waves travel
  about 1.7 times faster than S waves
• Secondary (S) waves
• Shake motion at right angles to their direction
  of travel
• Travel only through solids

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Seismic Waves
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Seismogram
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Time-Travel Graph (find distance to the
epicenter)
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Locating Earthquakes Source

• Epicenter is located using the difference in
  velocities of P and S waves
• Three station recordings required
• Time interval between first arrival of P wave and
  S wave at each station
• Travel-time graph is used to determine each
  stations distance to the epicenter
• Circle with radius equal to distance to epicenter
  is drawn around each station
• The point where all three circles intersect is
  the earthquake epicenter

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Epicenter located from 3 seismographs
Locating Earthquakes Source
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Locating Earthquakes Source

• Earthquake belts
• About 95 percent of the energy released by
  earthquakes originates in a few relatively narrow
  zones that wind around the globe
• Major earthquake zones include the Circum-Pacific
  belt, Mediterranean Sea region to the Himalayan
  complex, and the oceanic ridge system

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Distribution of Large Earthquakes, 1980 - 1990
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Locating Earthquakes Source

• Earthquake depths
• Earthquakes originate at depths ranging from 5 to
  nearly 700 kilometers
• Earthquake foci arbitrarily classified
• shallow (surface to 70 km),
• intermediate (between 70 and 300 km), and
• deep (over 300 kilometers)
• Definite patterns exist
• Shallow-focus oceanic ridge system
• Deep-focus circum-Pacific belt, deep-ocean
  trenches

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Measuring Earthquake Size

• Two earthquake size measurements
• Intensity a measure of the degree of earthquake
  shaking at a given locale based on the amount of
  damage
• Magnitude estimates the amount of energy
  released at the source of the earthquake
• Intensity scales
• Magnitude scales

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Measuring Earthquake Size

• Intensity scales
• Modified Mercalli Intensity Scale was developed
  using California buildings as its standard
• The drawback of intensity scales is that
  destruction may not be a true measure of the
  earthquakes actual severity

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Measuring Earthquake Size

• Magnitude scales
• Richter scale
• Based on the amplitude of the largest seismic
  wave recorded
• Accounts for the decrease in wave amplitude with
  increased distance
• Each unit of Richter magnitude increase
  corresponds to a tenfold increase in wave
  amplitude and a 32-fold energy increase

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Measuring Earthquake Size

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Measuring Earthquake Size
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Measuring Earthquake Size
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Measuring Earthquake Size

• Magnitudes scales
• Several Richter-like magnitude scales have
  been developed
• Moment magnitude was developed because none of
  the Richter-like magnitude scales adequately
  estimates the size of very large earthquakes
• Derived from the amount of displacement that
  occurs along a fault

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Earthquake Destruction

• Destruction from seismic vibrations
• Ground shaking
• Regions within 20 to 50 km of the epicenter will
  experience about the same intensity of ground
  shaking
• However, destruction varies considerably mainly
  due to the nature of the ground on which the
  structures are built
• Liquefaction of the ground
• Unconsolidated materials saturated with water
  turn into a mobile fluid
• Muddy flow

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1989 Loma PrietaSan Francisco, CA
Modified Mercalli Intensity Scale
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1989 Loma Prieta San Francisco, CA
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Liquefaction Damage1985 Mexico City
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Earthquake Destruction

• Destruction from seismic vibrations
• Seiches
• The rhythmic sloshing of water in lakes,
  reservoirs, and enclosed basins
• Waves can weaken reservoir walls and cause
  destruction
• Tsunamis, or seismic sea waves
• Destructive waves that are often inappropriately
  called tidal waves
• Result from vertical displacement along a fault
  located on the ocean floor
• Large undersea landslide triggered by an
  earthquake

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Earthquake Destruction

• Tsunamis, or seismic sea waves
• In the open ocean height is usually less than 1 m
  
• In shallower coastal waters the water piles up to
  heights that may exceed 30 m
• Can be very destructive
• Landslides and ground subsidence